Loaded waveguides



June 24, 1958 1.. B. MULLETT ET AL 2,340,788

LOADED WAVEGUIDES Filed Dec. 11, 1952 FIG. I.

IN VEN TORS.

LESLIE B. MULLETT BY BERNARD G. LOACH GEORGE L. ADAMS A TTOPNFY.

United States ate-nt Gfiice 2,840,788 Patented June 24, 1958 L ADE-WAVEGU E:

Leslie BadenMullett, Bernard GuestLoaeh, and George Lawson Adams,Malvern, Worcester, England, assignors, by mesne assignments, to theUaited States of Americans-represented by the United States AtomicEnergy Commission Applicafim pecember 11, 19 52, SerialNo, 325,306.

4 Claims. (Cl. 333-31) This invention relates toloaded, waveguides forthe propagation of electromagnetic waves with slow pl as evelocities.

In the specification of copending patent, application Serial No.236,932, filed July 16, 1951, now Patent No. 2,770,781, there isdiscloseda loading means for a waveguide comprising an elongated sheethaving inductive faces and adapted to be placed in a central plane, of arectangular waveguide parallel to the wider sides (the major medianplane), the waveguide being dimensioned so as to cut-off the simple Hmode,

The present invention is concerned with extensions and applications ofthe above mentioned disclosure According to the present invention arectangular waveguide dimensioned so as to cut-off the simple H mode atthe operating frequency is capacitatively loaded. Such loading mayconsist, for example, of corrugations in one or both walls of the guideor of transverse slots in a centrally arranged sheet. To be capacitativethe corrugations or slots require to have effective transversedimensions above cut-off i. e. greater than the width of the Waveguide.This may be achieved by making the corrugated portion of the guidephysically wider than the guide or by making the corrugations of thesame or less width than the guide and filling them with dielectric.Alternatively the corrugations may be ribbed. In the case of a slottedplate the slots may be made physically longer than the width of theguide by giving them a curved, inclined, or zig-zag configuration or bythickening the plate over a part or parts of its width as disclosed inthe specification of the aforesaid copending application. It will beunderstood that the equivalent of a slotted plate may be achieved bymeans of a series of parallel wires.

Reference is directed to the accompanying drawing wherein:

Fig. 1 illustrates a waveguide loading arrangement utilizing a series ofspaced loading members of rectangular cross section.

Fig. 2 illustrates a waveguide loading arrangement incorporating aseries of spaced loading members of circular cross section.

Fig. 3 illustrates a waveguide loading arrangement wherein a series ofapertured members load the waveguide.

A waveguide dimensioned to be above the H cutofi may be eitherinductively or capacitively loaded. To cutoff the H mode the waveguidemust be dimensioned to have a width less than 2 at the operatingfrequency. Various ways are available for achieving capacitive loadingin the waveguide and, consequently, reduce the phase velocity of theelectromagnetic wave transmitted by the waveguide, as will be moreclearly pointed out below.

The waveguide dimensioned to cut-off the H mode at the operatingfrequency may be provided with corrugations in one or both of the guidewalls. The corrugations may comprise pockets extending outward of theguide walls in the wide dimension walls spacedlongg tudinally along theguide and having the same width;dimensionas; the width of wide dimensionwalls. Or the PQGkets maybe of greater width than the wider dimen onwalls, In both cases. the pockets may be filled with-di: electricmaterial. The dielectric acts to render each corrugation, in effect, ashort length of waveguide, which is above'cut-oif, that is dimensionedto have anetfective electrical length of greater than /2 thewavelengthat the operating frequency. It will be appreciated that; the slotsmay beless than the width of the guide if'the. constant of the dielectricis-high enough. Unlike'th'econventional corrugated waveguide practicethedeptlr. of the corrugation is greater than 4.

Another method'of loading the waveguide corrugation is to provide ridgesalong the centres of the wider sides of the Waveguides constituted bythe corrugation. These ridges are part of the corrugated portion of thewaveguide and extend along the depth dimension of the corrugationpocket.

The loading means, instead of forming corrugation pockets exterior tothe waveguide walls, may comprise a series of conductive slats extendingdown part way from one of the wider walls of the waveguide in atransverse plane to the guide and perpendicular to the plan Qfjthenarrower guide walls. As in the externally corrugated loading means thepocket may be filled with dielectric, to further load the guide, or thewalls of the corrugated'extension may have ribs or ridges to provide theloading effect.

The embodiment as shown in Fig. 3 may be originally taken to comprise arectangular waveguide with a series of slats disposed centrally withinthe guide Walls. 7 Dielectric filling or ridges on the slats may beutilized to add to the loading, thereby increasing the loading effect ofthe slats as a result of the increase in effective electrical wavelengthof the loading structure disposed between the waveguide walls. The depthof the slats (the dimension along the narrower wall of the waveguide)tends to zero as the loaded guide wavelength is reduced to a value muchless than the air wavelength. Hence, if the depth of the slats tend tozero, they may be replaced by a slotted plate having capacitative slots.For example, if the slats mentioned above are reduced in depth at theirends to leave a circular enlargement, ring or annular portion around ahole in the slat, as shown in Fig. 3, the series of slats may then beregarded as slotted plates with a semi-circular ridge along the centreof each filce, these ridges providing capacitative loading for the sots.

A. practical embodiment of an S-band loaded waveguide of the form shownin Fig. 3 and suitable for proton acceleration is of the followingdimensions:

Cross section of guide cm 4.5 x 3.4 Depth of slats 4 (or thickness ofplate) in .125 Spacing of slats 4 (or width of slots) in .125 Diameterof hole in .394 Diameter of ring in .625

the slots physically longer than half a wavelength (in air) by, forexample, inclining the slots in a transverse plane of the waveguide orshaping the slots as shown in Figs. 1 and 2. Using the inclined slotsthe displacement along the waveguide of the opposite ends of each slotmust be small compared with the loaded guide wave length.

In Figs. 1 and 2 the length of the slots is made longer than the widedimension of the guide by, in effect, longitudinally corrugating theplate. The slot may be made from a rectangular piece of metal stock, ora piece of circular metal stock as shown in Fig. 2. Instead, however, ofactually slotting a plate a series of spaced parallel round wires 2 or 3are used and the wires are corrugated (zig-zagged) in the transverseplane of the guide.

One practical construction based on Fig. 2 for operation in S-band wasdimensioned as follows:

Cross section of guide 4.5 cm. x 3.4 cm.

Diameter of wires 3 .080 in.

Pitch of wires 3 .16 in.

Shape of corrugations 4 T. P. I. Whitworth thread form.

As in the previous dimensioned example, the above example cuts oil the Hmode when operating in the S- band and is capacitatively loaded, givinga negative phase velocity. The loading is such that for an airwavelength of 10.4 cm. the guide wavelength is 2.1 cm. This example ofthe invention has particular application in connection with micro-waveoscillators.

We claim:

1. A rectangular waveguide having narrower and wider pairs of walls,means to excite the waveguide at a frequency below the cutofi frequencyof the waveguide, an electrical conductor connected between the narrowerpair of waveguide walls in the major median plane from directly oppositepoints on said walls, at least a portion of said conductor beingdisplaced from the axis of the line connecting said directly oppositepoints, the electrical length of said conductor being greater thanonehalf wavelength at said frequency. 1

2. A rectangular Waveguide as claimed in claim 1, wherein said conductoris around wire and said displaced portion comprises a series ofcorrugations.

3. A rectangular waveguide as claimed in claim 1, wherein the conductoris of rectangular cross section, said displaced portion comprising aseries of zig-zag corrugations.

4. A rectangular waveguide as claimed in claim 1, wherein said conductorhas an annular portion perpendicular to and extending above and belowthe major median plane and supported on opposite sides by conductor armsextending and connected to said directly opposite points.

References Cited in the file of this patent UNITED STATES PATENTS2,527,477 Clapp Oct. 24, 1950 2,567,748 White Sept. 11, 1951 2,573,012Gutton Oct. 30, 1951 2,576,186 Malter Nov. 27, 1951 2,661,441 MuellerDec. 1, 1953 2,708,236 Pierce May 10, 1955

